The present invention relates to technology for driving an ink jet recording head in which a piezoelectric vibrator is used as an actuator.
Vertical mode piezoelectric vibrators and flexural vibration mode piezoelectric vibrators are examples of high-speed drive actuators used in ink jet recording heads. High-speed drive actuators are formed, in part, of an elastic plate. Such actuators are used in ink jet recording heads of the type described below.
Such ink jet recording heads include piezoelectric vibrators, pressure producing chambers, and nozzle openings. In particular, an ink jet recording head may draw ink from an ink source by using a sucking force. The ink so drawn enters a pressure producing chamber. The pressure producing chamber communicates with a nozzle opening. The pressure producing chamber can be expanded and contracted. The expansion and contraction of the pressure producing chamber is performed by a piezoelectric vibrator.
The expansion and contraction of the pressure producing chamber by the piezoelectric vibrator is what causes the sucking force which draws ink into the pressure producing chamber. The expansion and contraction of the pressure producing chamber by the piezoelectric vibrator is also what causes the expulsion of a desired ink droplet through the nozzle opening.
A vertical mode piezoelectric vibrator is formed by laminating a piezoelectric material and a conductive layer one upon another. A flexural mode piezoelectric vibrator is formed by arranging a piezoelectric vibrating thin layer on a surface of a vibrating plate. Such a thin film may be formed, for example, by sputtering or vapor deposition.
Such a piezoelectric vibrator has only a small area in contact with the vibrating plate, and is capable of being driven at high speed. This sort of piezoelectric vibrator is advantageous in that it permits the high density arrangement of the pressure producing chambers. As a result, high-resolution and high-speed printing can be achieved.
The high density arrangement of the pressure producing chambers is not, however, without its problems. One problem involves unwanted vibrations.
To explain, it is important first to define some terms which will be used throughout this description. These terms are "desired pressure producing chamber", "desired piezoelectric vibrator", "physically adjacent chamber", "physically adjacent piezoelectric vibrator", "undesired adjacent pressure producing chamber", and "undesired adjacent piezoelectric vibrator".
For the purposes of this description, the term "desired pressure producing chamber" refers to a pressure producing chamber that presently should be driven to produce an ink droplet. Whether a pressure producing chamber presently should be driven depends, normally, on the print data. The piezoelectric vibrator of a desired pressure producing chamber shall be referred to as a "desired piezoelectric vibrator".
For the purposes of this description, the term "physically adjacent chamber" means a pressure producing chamber that is physically adjacent to another pressure producing chamber. Whether a pressure producing chamber is a physically adjacent chamber of another pressure producing chamber depends on the physical layout of the pressure producing chambers. The piezoelectric vibrator of a physically adjacent vibrator shall be referred to as a "physically adjacent piezoelectric vibrator".
In this description, the term "undesired adjacent pressure producing chamber" refers to a physically adjacent chamber of a desired pressure producing chamber and, in particular, one which presently should not be driven to produce an ink droplet. Thus, a undesired adjacent pressure producing chamber not only is a physically adjacent chamber, but also is a chamber from which no present jetting of an ink-droplet is desirable. Whether a physically adjacent chamber presently should be driven depends, normally, on the print data. The piezoelectric vibrator of an undesired adjacent pressure producing chamber shall be referred to as an "undesired adjacent piezoelectric vibrator".
As mentioned above, one problem with an ink jet recording head that has pressure producing chambers arranged at a high density is that the vibration of a desired pressure producing chamber may propagate as far as an undesired adjacent pressure producing chamber. The vibrations thus propagate may cause an ink droplet to be jetted from the undesired adjacent pressure producing chamber. This jetting of an ink droplet from an undesired adjacent pressure producing chamber is known as the crosstalk phenomenon. The crosstalk phenomenon is a problem because it results in the jetting of an ink droplet independently of the application of a drive signal. In other words, even though the undesired adjacent piezoelectric vibrator of the undesired adjacent pressure producing chamber is not driven, an ink droplet may nevertheless be jetted.
To meet the need for high-density printing, an ink jet recording head not only might provide a high density arrangement of pressure producing chambers, but also might use a smaller amount of ink for forming its ink droplets. Such a printer, to provide proper printed output, must take care of the crosstalk phenomenon. The crosstalk phenomenon problem occurs especially easily in such an ink jet recording head, however, because the compliance of a pressure producing chamber is controlled to be a small value.
A more detailed explanation of this problem is now made with reference to FIGS. 10 and 11. In FIG. 10, piezoelectric vibrators D and F are, at the same time, desired piezoelectric vibrators. In other words, piezoelectric vibrators D and F are both presently to be driven in accordance with the print data so that ink droplets will be jetted from the pressure producing chambers to which piezoelectric vibrators D and F correspond. The pressure producing chambers to which piezoelectric vibrators D and F correspond thus are desired pressure producing chambers. Both piezoelectric vibrators D and F have in common, as an undesired adjacent piezoelectric vibrator, piezoelectric vibrator E. In other words, piezoelectric vibrator E is not presently to be driven, and no ink droplet from the pressure producing chamber to which piezoelectric vibrator E corresponds is desired. Thus, the pressure producing chamber to which piezoelectric vibrator E corresponds is an undesired adjacent pressure producing chamber.
FIG. 10 thus shows a vibrating unit with a plurality of piezoelectric vibrators B to G. Piezoelectric vibrators D and F are presently desired piezoelectric vibrators, and piezoelectric vibrator E, with respect to each of piezoelectric vibrators D and F, is an undesired adjacent piezoelectric vibrator. Piezoelectric vibrators B to G are fixed to a highly rigid fixing board A. These piezoelectric vibrators are fixed to the fixing board so that each piezoelectric vibrator corresponds to a respective pressure producing chamber. In other words, each of the plurality of piezoelectric vibrators is operationally disposed with respect to a corresponding pressure producing chamber.
Piezoelectric vibrators D and F are presently to be driven by drive signals so that the aforementioned expansion and contraction of their respective pressure producing chambers may be accomplished. In particular, the piezoelectric vibrators may be driven by drive signals that have a trapezoidal shape as shown in FIG. 11(I). Drive signals having the general shape as shown in FIG. 11(I) may be referred to as trapezoidal drive signals. A first part of the trapezoidal drive signal in FIG. 11(I) is characterized by a rising slope. The effect of this first part of the trapezoidal drive signal may be referred to as "charging". A second part is characterized by a level signal. The effect of this part of the trapezoidal drive signal may be referred to as "holding". A third part of the drive signal is characterized by a falling slope, and the corresponding effect may be referred to as "discharging".
When trapezoidal drive signals such as that shown in FIG. 11(I) are applied to desired piezoelectric vibrators D and F, but not applied to undesired adjacent piezoelectric vibrator E, the corresponding pressure producing chambers behave according to the following description.
Reference is now made to FIG. 11(II). This figure shows the volume of a pressure producing chamber. The horizontal line is a reference line which represents the volume of the pressure producing chamber when the pressure producing chamber is neither expanded nor contracted. The data points below the horizontal reference line represent contraction. The further from the horizontal reference line a data point is, the more the pressure producing chamber is contracted. Likewise, data points above the horizontal reference line represent expansion of the pressure producing chamber. As will be appreciated, the wavy line in FIG. 11(II) represents the expansion and contraction of a pressure producing chamber over time.
For convenience, the following description may state that a piezoelectric vibrator contracts or expands. This is merely a shorthand way of stating that the piezoelectric vibrator is driven in a certain manner which causes the corresponding pressure producing chamber to experience contraction or expansion.
During the first part of the trapezoidal drive signal (i.e., during charging), the piezoelectric vibrators D and F contract as shown in FIG. 11(II). During the second part of the trapezoidal drive signal (i.e., during holding), the drive signal is held at a predetermined voltage. When charging stops and holding begins, natural vibrations are caused. In other words, when the piezoelectric vibrators stop contracting and are held, natural vibrations result. These natural vibrations may be referred to as free vibrations or as first natural vibrations.
Holding of piezoelectric vibrators D and F lasts for a predetermined period of time. In order to jet ink droplets after a predetermined time has elapsed, the charges stored in the piezoelectric vibrators D and F are discharged so as to expand these piezoelectric vibrators. After the ink droplets have been jetted out, the piezoelectric vibrators D and F start vibrating naturally again. The natural vibrations caused after the jetting of the ink droplets may be referred to as latter free vibrations or as second natural vibrations.
On the other hand, the undesired adjacent piezoelectric vibrator E, to which no drive signal has been applied, receives vibrations through the fixing board A. The undesired adjacent piezoelectric vibrator E receives not only the free vibrations, but also the latter free vibrations created with the jetting of the ink droplets from desired piezoelectric vibrators D and F.
As a result, the undesired adjacent piezoelectric vibrator E has the amplitude of a vibration thereof amplified as shown in FIG. 11 (III) due to interference between the vibrations at the time of charging and the vibrations after the ink droplets have been jetted out. The amplitude of the vibration of the piezoelectric vibrator E caused by the propagation is in the order of 10% of the maximum amplitude of the vibrations of the desired piezoelectric vibrators D and F. However, if the vibration of the piezoelectric vibrator E lasts for a plurality of cycles, e.g., for three cycles or more, then the vibration of the meniscus of the nozzle opening corresponding to the piezoelectric vibrator E is amplified, which in turn causes an ink droplet undesirably to be jetted out.